REGISTRATION RIGHTS AGREEMENT


          This  Registration  Rights  Agreement  (this  "Agreement") is made and
entered  into as of  February  24,  2003,  by and among  Aphton  Corporation,  a
Delaware   corporation   (the   "Company"),   and  Mainfield   Enterprises  Inc.
("Purchaser").

          This Agreement is made pursuant to the Securities  Purchase Agreement,
dated as of the date hereof among the Company and the Purchaser  (the  "Purchase
Agreement").

          The Company and the Purchaser hereby agree as follows:

     1.  Definitions.  Capitalized  terms used and not otherwise  defined herein
that are defined in the Purchase  Agreement  shall have the meanings  given such
terms in the Purchase Agreement. As used in this Agreement,  the following terms
shall have the following meanings:

          "Common  Stock" shall mean the common stock of the Company,  par value
$0.001 per share.

          "Effective  Date"  means the  date,  if any,  that  such  Registration
Statement is first declared effective by the Commission.

          "Effectiveness  Date" means, the earlier of (1) the 90th day following
the Closing Date,  and (2) the fifth Trading Day following the date on which the
Company is notified by the Commission that the  Registration  Statement will not
be reviewed or is no longer subject to further review and comments.

          "Effectiveness  Period"  shall have the  meaning  set forth in Section
2(a).

          "Exchange Act" means the Securities Exchange Act of 1934, as amended.

          "Filing Date" means the 30th day following the Closing Date.

          "Holder" or "Holders" means the holder or holders, as the case may be,
from time to time of Registrable Securities.

          "Indemnified Party" shall have the meaning set forth in Section 5(c).

          "Indemnifying Party" shall have the meaning set forth in Section 5(c).

          "Liquidated  Damages  Product"  shall  have the  meaning  set forth in
Section 2(b).

          "Losses" shall have the meaning set forth in Section 5(a).
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          "Proceeding" means an action, claim, suit, investigation or proceeding
(including,  without limitation, an investigation or partial proceeding, such as
a deposition), whether commenced or threatened.

          "Prospectus" means the prospectus included in a Registration Statement
(including,  without  limitation,  a prospectus  that  includes any  information
previously omitted from a prospectus filed as part of an effective  registration
statement in reliance upon Rule 430A  promulgated  under the Securities Act), as
amended or supplemented by any prospectus supplement,  with respect to the terms
of the  offering of any  portion of the  Registrable  Securities  covered by the
Registration  Statement,  and  all  other  amendments  and  supplements  to  the
Prospectus,  including post-effective  amendments, and all material incorporated
by reference or deemed to be incorporated by reference in such Prospectus.

          "Registrable  Securities"  means the shares of Common  Stock  issuable
upon exercise of the Warrants.

          "Registration  Statement" means the registration statement required to
be filed hereunder including the Prospectus,  amendments and supplements to such
registration   statement  or  Prospectus,   including  pre-  and  post-effective
amendments,  all exhibits thereto, and all material incorporated by reference or
deemed to be incorporated by reference in such registration statement.

          "Rule 144" means Rule 144  promulgated by the  Commission  pursuant to
the  Securities  Act,  as such Rule may be  amended  from  time to time,  or any
similar  rule  or  regulation   hereafter   adopted  by  the  Commission  having
substantially the same effect as such Rule.

          "Rule 415" means Rule 415  promulgated by the  Commission  pursuant to
the  Securities  Act,  as such Rule may be  amended  from  time to time,  or any
similar  rule  or  regulation   hereafter   adopted  by  the  Commission  having
substantially the same effect as such Rule.

          "Rule 424" means Rule 424  promulgated by the  Commission  pursuant to
the  Securities  Act,  as such Rule may be  amended  from  time to time,  or any
similar  rule  or  regulation   hereafter   adopted  by  the  Commission  having
substantially the same effect as such Rule.

          "Securities Act" means the Securities Act of 1933, as amended.

          "Special Counsel" means Bryan Cave LLP.

          "Warrants"  means the  Warrants  issued or issuable  to the  Purchaser
under the Purchase Agreement.

     2.    Registration.

          (a) On or prior to the Filing Date, the Company shall prepare and file
with  the  Commission  a  Registration  Statement  covering  the  resale  of all
Registrable Securities for an offering to be made on a continuous basis pursuant
to Rule 415.  The  Registration  Statement  shall be on Form S-3  (except if the
Company is not then eligible to register for resale the  Registrable  Securities
on Form S-3,  in which case such  registration  shall be on another

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appropriate form in accordance  herewith) and shall contain (except if otherwise
directed by the Holders) the "Plan of Distribution"  attached hereto as Annex A.
The Company  shall cause the  Registration  Statement  to be declared  effective
under the  Securities  Act as soon as possible but, in any event,  no later than
the Effectiveness  Date, and shall use its best efforts to keep the Registration
Statement  continuously  effective under the Securities Act until the date which
is two  years  after  the  date  that the  Registration  Statement  is  declared
effective by the Commission or such earlier date when all Registrable Securities
covered by such  Registration  Statement  have been sold or may be sold  without
volume restrictions  pursuant to Rule 144(k) as determined by the counsel to the
Company  pursuant to a written  opinion  letter to such  effect,  addressed  and
acceptable  to the  Company's  transfer  agent  and the  affected  Holders  (the
"Effectiveness Period").

          (b) If: (i) the Registration Statement is not filed on or prior to the
Filing Date (if the Company files such Registration  Statement without affording
the Holder the  opportunity  to review and  comment on the same as  required  by
Section  3(a) hereof,  the Company  shall not be deemed to have  satisfied  this
clause (i)), or (ii) the Company fails to file with the Commission a request for
acceleration in accordance  with Rule 461 promulgated  under the Securities Act,
within five Trading Days of the date that the Company is notified  (orally or in
writing, whichever is earlier) by the Commission that the Registration Statement
will not be "reviewed," or not subject to further  review,  or (iii) the Company
fails to respond to any comments  made by the  Commission  within ten days after
the receipt of such comments, or (iv) after its Effective Date, the Registration
Statement  ceases to be effective and available to the Holders  thereunder as to
all  Registrable  Securities to which it is required to relate (whether upon the
delivery of an Advice  pursuant to Section 6(d) or  otherwise) at any time prior
to the expiration of its  Effectiveness  Period without being  succeeded  within
fifteen  Trading  Days by an amendment  to such  Registration  Statement or by a
subsequent  Registration  Statement  filed with and  declared  effective  by the
Commission,  or (v) an amendment to the  Registration  Statement is not filed by
the Company with the Commission  within fifteen Trading Days of the Commission's
notifying  the  Company  that  such  amendment  is  required  in  order  for the
Registration Statement to be declared effective,  or (vi) the exercise rights of
the Holders pursuant to the Warrants are suspended for any reason,  or (vii) the
Registration  Statement shall not be declared  effective by the Commission on or
prior to the Effectiveness Date (any such failure or breach being referred to as
an "Event," and for purposes of clause (i), (vi) or (vii) the date on which such
Event occurs, or for purposes of clause (ii) the date on which such five Trading
Day period is exceeded,  or for purposes of clause (iii) the date which such ten
day-period  is  exceeded,  or for purposes of clauses (iv) or (v) the date which
such fifteen Trading Day-period is exceeded, being referred to as "Event Date"),
then, in addition to any other rights available to the Holders: (x) on each such
Event Date the Company shall pay to each Holder an amount in cash, as liquidated
damages  and not as a  penalty,  equal to 1% of the  product  of the  number  of
Warrant  Shares  then  issuable  upon  exercise  in full of the  Warrant and the
greater of (1) the closing sales price of the Common Stock on the Original Issue
Date and (2) the closing  sales price of the Common Stock as of such  applicable
Event Date ("Liquidated  Damages Product");  and (y) on each monthly anniversary
of each such Event Date  thereof  (if the  applicable  Event shall not have been
cured by such date) until the applicable  Event is cured,  the Company shall pay
to each Holder an amount in cash,  as  liquidated  damages and not as a penalty,
equal to 2% of the Liquidated  Damages Product.  If the Company fails to pay any
liquidated  damages pursuant to this Section in full within seven days after the
date payable,  the Company will pay interest  thereon at a rate of 18% per annum
(or such

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lesser  maximum  amount that is permitted to be paid by  applicable  law) to the
Holder,  accruing daily from the date such liquidated damages are due until such
amounts, plus all such interest thereon, are paid in full.

     3.   Registration Procedures

          In connection with the Company's  registration  obligations hereunder,
the Company shall:

          (a) Not  less  than  two  Trading  Days  prior  to the  filing  of the
Registration  Statement or any related Prospectus or any amendment or supplement
thereto, the Company shall, (i) furnish to the Holders and their Special Counsel
copies  of  all  such  documents  proposed  to  be  filed  (including  documents
incorporated  or deemed  incorporated  by  reference)  which  documents  will be
subject to the review of such Holders and their Special Counsel,  and (ii) cause
its officers and directors, counsel and independent certified public accountants
to respond to such inquiries as shall be necessary, in the reasonable opinion of
respective counsel to conduct a reasonable  investigation  within the meaning of
the Securities Act. The Company shall not file the Registration Statement or any
such Prospectus or any amendments or supplements thereto to which the Holders of
a  majority  of the  Registrable  Securities  and their  Special  Counsel  shall
reasonably object in good faith.

          (b)  (i)  Prepare  and  file  with  the  Commission  such  amendments,
including  post-effective  amendments,  to the  Registration  Statement  and the
Prospectus  used  in  connection  therewith  as may be  necessary  to  keep  the
Registration  Statement  continuously effective as to the applicable Registrable
Securities for the Effectiveness Period and prepare and file with the Commission
such  additional  Registration  Statements in order to register for resale under
the Securities  Act all of the  Registrable  Securities;  (ii) cause the related
Prospectus to be amended or supplemented by any required Prospectus  supplement,
and as so  supplemented  or  amended  to be filed  pursuant  to Rule 424;  (iii)
respond as promptly as reasonably possible, and in any event within ten days, to
any comments  received  from the  Commission  with  respect to the  Registration
Statement  or any  amendment  thereto  and, as promptly as  reasonably  possible
provide the Holders true and complete copies of all  correspondence  from and to
the Commission  relating to the Registration  Statement;  and (iv) comply in all
material respects with the provisions of the Securities Act and the Exchange Act
with respect to the  disposition of all  Registrable  Securities  covered by the
Registration  Statement  during the  applicable  period in  accordance  with the
intended  methods  of  disposition  by the  Holders  thereof  set  forth  in the
Registration Statement as so amended or in such Prospectus as so supplemented.

          (c) Notify the Holders of Registrable  Securities to be sold and their
Special  Counsel as promptly as reasonably  possible (and, in the case of (i)(A)
below,  not less than three Trading Days prior to such filing) and (if requested
by any such Person) confirm such notice in writing no later than one Trading Day
following  the day (i)(A) when a  Prospectus  or any  Prospectus  supplement  or
post-effective  amendment to the Registration Statement is proposed to be filed;
(B) when the Commission notifies the Company whether there will be a "review" of
such Registration  Statement and whenever the Commission  comments in writing on
such Registration  Statement (the Company shall provide true and complete copies
thereof and all written responses thereto to each of the Holders);  and (C) with
respect to the Registration

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Statement or any post-effective  amendment,  when the same has become effective;
(ii) of any request by the Commission or any other Federal or state governmental
authority  for  amendments  or  supplements  to the  Registration  Statement  or
Prospectus  or  for  additional  information;  (iii)  of  the  issuance  by  the
Commission of any stop order  suspending the  effectiveness  of the Registration
Statement covering any or all of the Registrable Securities or the initiation of
any  Proceedings  for that  purpose;  (iv) of the  receipt by the Company of any
notification  with respect to the suspension of the  qualification  or exemption
from  qualification  of  any of  the  Registrable  Securities  for  sale  in any
jurisdiction,  or the  initiation  or  threatening  of any  Proceeding  for such
purpose;  and (v) of the  occurrence  of any event or passage of time that makes
the financial  statements included in the Registration  Statement ineligible for
inclusion  therein  or any  statement  made  in the  Registration  Statement  or
Prospectus or any document  incorporated or deemed to be incorporated therein by
reference  untrue in any material  respect or that requires any revisions to the
Registration  Statement,  Prospectus or other  documents so that, in the case of
the  Registration  Statement or the Prospectus,  as the case may be, it will not
contain any untrue  statement  of a material  fact or omit to state any material
fact required to be stated therein or necessary to make the statements  therein,
in light of the circumstances under which they were made, not misleading.

          (d) Use its best  efforts  to avoid the  issuance  of,  or, if issued,
obtain the  withdrawal  of (i) any order  suspending  the  effectiveness  of the
Registration  Statement,  or  (ii)  any  suspension  of  the  qualification  (or
exemption from  qualification) of any of the Registrable  Securities for sale in
any jurisdiction, at the earliest practicable moment.

          (e) Furnish to each Holder and their Special Counsel,  without charge,
at least one conformed  copy of each  Registration  Statement and each amendment
thereto,   including   financial   statements  and   schedules,   all  documents
incorporated or deemed to be incorporated therein by reference, and all exhibits
to the extent requested by such Person (including those previously  furnished or
incorporated by reference)  promptly after the filing of such documents with the
Commission.

          (f) Promptly deliver to each Holder and their Special Counsel, without
charge, as many copies of the Prospectus or Prospectuses (including each form of
prospectus)  and each  amendment  or  supplement  thereto  as such  Persons  may
reasonably  request.  The Company hereby  consents to the use of such Prospectus
and each  amendment  or  supplement  thereto by each of the  selling  Holders in
connection with the offering and sale of the Registrable  Securities  covered by
such Prospectus and any amendment or supplement thereto.

          (g) Prior to any public  offering of Registrable  Securities,  use its
best  efforts to register or qualify or cooperate  with the selling  Holders and
their Special Counsel in connection with the registration or  qualification  (or
exemption  from  such   registration  or   qualification)  of  such  Registrable
Securities  for offer and sale  under  the  securities  or Blue Sky laws of such
jurisdictions  within the United  States as any Holder  requests in writing,  to
keep each such registration or qualification (or exemption  therefrom) effective
during  the  Effectiveness  Period  and to do any and all  other  acts or things
necessary or advisable to enable the  disposition in such  jurisdictions  of the
Registrable Securities covered by a Registration  Statement;  provided, that the
Company  shall not be  required  to  qualify  generally  to do  business  in any
jurisdiction  where it is

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<PAGE>


not then so  qualified  or subject the Company to any  material  tax in any such
jurisdiction where it is not then so subject.

          (h) Cooperate  with the Holders to facilitate  the timely  preparation
and delivery of certificates representing Registrable Securities to be delivered
to a transferee pursuant to a Registration  Statement,  which certificates shall
be free, to the extent permitted by the Purchase  Agreement,  of all restrictive
legends,  and to enable such Registrable  Securities to be in such denominations
and registered in such names as any such Holders may request.

          (i) Upon the occurrence of any event  contemplated by Section 3(c)(v),
as promptly as reasonably possible, prepare a supplement or amendment, including
a post-effective amendment, to the Registration Statement or a supplement to the
related  Prospectus or any document  incorporated  or deemed to be  incorporated
therein  by  reference,  and file  any  other  required  document  so  that,  as
thereafter  delivered,  neither the  Registration  Statement nor such Prospectus
will contain an untrue  statement of a material fact or omit to state a material
fact required to be stated therein or necessary to make the statements  therein,
in light of the circumstances under which they were made, not misleading.

          (j)  Comply  with  all  applicable   rules  and   regulations  of  the
Commission.

          (k) The  Company  may require  each  selling  Holder to furnish to the
Company a  certified  statement  as to the  number  of  shares  of Common  Stock
beneficially  owned by such Holder  and, if  requested  by the  Commission,  the
controlling person thereof.

     4. Registration Expenses. All fees and expenses incident to the performance
of or  compliance  with  this  Agreement  by the  Company  shall be borne by the
Company  whether or not any  Registrable  Securities  are sold  pursuant  to the
Registration  Statement.  The fees and  expenses  referred  to in the  foregoing
sentence shall include, without limitation, (i) all registration and filing fees
(including,  without  limitation,  fees and expenses (A) with respect to filings
required to be made with Nasdaq National Market or any other Trading Market, and
(B) in compliance  with  applicable  state  securities  or Blue Sky laws),  (ii)
printing  expenses   (including,   without  limitation,   expenses  of  printing
certificates  for  Registrable  Securities and of printing  prospectuses  if the
printing of prospectuses is reasonably requested by the holders of a majority of
the  Registrable  Securities  included  in the  Registration  Statement),  (iii)
messenger,  telephone  and delivery  expenses,  (iv) fees and  disbursements  of
counsel for the Company, (v) Securities Act liability insurance,  if the Company
so desires  such  insurance,  and (vi) fees and  expenses  of all other  Persons
retained by the Company in connection with the  consummation of the transactions
contemplated  by this Agreement.  In addition,  the Company shall be responsible
for all of its internal expenses incurred in connection with the consummation of
the transactions contemplated by this Agreement (including,  without limitation,
all salaries and expenses of its  officers  and  employees  performing  legal or
accounting  duties),  the expense of any annual  audit and the fees and expenses
incurred in  connection  with the listing of the  Registrable  Securities on any
securities exchange as required hereunder.

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     5. Indemnification

          (a) Indemnification by the Company. The Company shall, notwithstanding
any termination of this Agreement,  indemnify and hold harmless each Holder, the
officers,  directors,  agents,  brokers  (including  brokers  who offer and sell
Registrable  Securities  as  principal as a result of a pledge or any failure to
perform under a margin call of Common Stock),  investment advisors and employees
of each of them, each Person who controls any such Holder (within the meaning of
Section 15 of the  Securities  Act or Section  20 of the  Exchange  Act) and the
officers,  directors,  agents and employees of each such controlling  Person, to
the fullest  extent  permitted by  applicable  law, from and against any and all
losses,  claims,  damages,  liabilities,  costs (including,  without limitation,
reasonable  costs of preparation  and reasonable  attorneys'  fees) and expenses
(collectively,  "Losses"), as incurred, arising out of or relating to any untrue
or alleged  untrue  statement of a material fact  contained in the  Registration
Statement,  any  Prospectus  or any form of  prospectus  or in any  amendment or
supplement  thereto  or in any  preliminary  prospectus,  or  arising  out of or
relating to any omission or alleged  omission of a material  fact required to be
stated therein or necessary to make the  statements  therein (in the case of any
Prospectus  or form  of  prospectus  or  supplement  thereto,  in  light  of the
circumstances under which they were made) not misleading,  except to the extent,
but only to the extent,  that (1) such untrue  statements or omissions are based
solely  upon  information  regarding  such  Holder  furnished  in writing to the
Company by such Holder  expressly  for use  therein,  or to the extent that such
information  relates  to  such  Holder  or  such  Holder's  proposed  method  of
distribution of Registrable  Securities and was reviewed and expressly  approved
in writing by such Holder expressly for use in the Registration Statement,  such
Prospectus or such form of Prospectus or in any amendment or supplement  thereto
(it being  understood  that the  Holder  has  approved  Annex A hereto  for this
purpose) or (2) in the case of an occurrence  of an event of the type  specified
in Section  3(c)(ii)-(v),  the use by such Holder of an  outdated  or  defective
Prospectus  after the  Company  has  notified  such  Holder in writing  that the
Prospectus  is outdated or defective  and prior to the receipt by such Holder of
the Advice  contemplated  in Section 5(e).  The Company shall notify the Holders
promptly of the institution,  threat or assertion of any Proceeding of which the
Company  is  aware in  connection  with the  transactions  contemplated  by this
Agreement.

          (b) Indemnification by Holders.  Each Holder shall,  severally and not
jointly,  indemnify  and hold  harmless the Company,  its  directors,  officers,
agents and employees,  each Person who controls the Company  (within the meaning
of Section 15 of the Securities Act and Section 20 of the Exchange Act), and the
directors,  officers,  agents or employees of such controlling  Persons,  to the
fullest  extent  permitted by applicable  law,  from and against all Losses,  as
incurred,  arising solely out of or based solely upon: (x) such Holder's failure
to comply with the prospectus delivery requirements of the Securities Act or (y)
any untrue statement of a material fact contained in any Registration Statement,
any  Prospectus,  or any form of  prospectus,  or in any amendment or supplement
thereto,  or  arising  solely  out of or based  solely  upon any  omission  of a
material fact required to be stated  therein or necessary to make the statements
therein not misleading to the extent,  but only to the extent,  that such untrue
statement or omission is contained in any information so furnished in writing by
such Holder to the  Company  specifically  for  inclusion  in such  Registration
Statement or such Prospectus or to the extent that (1) such untrue statements or
omissions are based solely upon  information  regarding such Holder furnished in
writing to the  Company by such  Holder  expressly  for use  therein,  or to

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the  extent  that such  information  relates  to such  Holder  or such  Holder's
proposed method of  distribution of Registrable  Securities and was reviewed and
expressly  approved  in  writing  by  such  Holder  expressly  for  use  in  the
Registration Statement (it being understood that the Holder has approved Annex A
hereto for this purpose),  such  Prospectus or such form of Prospectus or in any
amendment or supplement  thereto or (2) in the case of an occurrence of an event
of the type  specified  in Section  3(c)(ii)-(v),  the use by such  Holder of an
outdated or defective  Prospectus  after the Company has notified such Holder in
writing that the Prospectus is outdated or defective and prior to the receipt by
such Holder of the Advice  contemplated  in Section  6(d). In no event shall the
liability of any selling  Holder  hereunder be greater in amount than the dollar
amount  of the  net  proceeds  received  by such  Holder  upon  the  sale of the
Registrable Securities giving rise to such indemnification obligation.

          (c) Conduct of Indemnification Proceedings. If any Proceeding shall be
brought or asserted  against  any Person  entitled to  indemnity  hereunder  (an
"Indemnified  Party"),  such Indemnified  Party shall promptly notify the Person
from whom  indemnity is sought (the  "Indemnifying  Party") in writing,  and the
Indemnifying Party shall assume the defense thereof, including the employment of
counsel reasonably  satisfactory to the Indemnified Party and the payment of all
fees and expenses  incurred in connection with defense thereof;  provided,  that
the failure of any  Indemnified  Party to give such notice shall not relieve the
Indemnifying Party of its obligations or liabilities pursuant to this Agreement,
except (and only) to the extent that it shall be finally  determined  by a court
of  competent  jurisdiction  (which  determination  is not  subject to appeal or
further  review)  that  such  failure  shall  have  proximately  and  materially
adversely prejudiced the Indemnifying Party.

          An Indemnified  Party shall have the right to employ separate  counsel
in any such Proceeding and to participate in the defense  thereof,  but the fees
and expenses of such counsel shall be at the expense of such  Indemnified  Party
or Parties unless:  (1) the Indemnifying Party has agreed in writing to pay such
fees and  expenses;  (2) the  Indemnifying  Party shall have failed  promptly to
assume  the  defense  of  such  Proceeding  and  to  employ  counsel  reasonably
satisfactory to such Indemnified Party in any such Proceeding;  or (3) the named
parties to any such Proceeding  (including any impleaded  parties)  include both
such Indemnified  Party and the Indemnifying  Party, and such Indemnified  Party
shall have been  advised by counsel  that a conflict  of  interest  is likely to
exist if the same  counsel  were to  represent  such  Indemnified  Party and the
Indemnifying  Party (in which  case,  if such  Indemnified  Party  notifies  the
Indemnifying  Party in writing that it elects to employ separate  counsel at the
expense of the Indemnifying  Party,  the  Indemnifying  Party shall not have the
right to assume the defense  thereof and such counsel shall be at the expense of
the  Indemnifying  Party).  The  Indemnifying  Party shall not be liable for any
settlement of any such Proceeding  effected without its written  consent,  which
consent shall not be unreasonably withheld. No Indemnifying Party shall, without
the prior written consent of the Indemnified Party, effect any settlement of any
pending Proceeding in respect of which any Indemnified Party is a party,  unless
such settlement includes an unconditional release of such Indemnified Party from
all liability on claims that are the subject matter of such Proceeding.

          All fees and expenses of the Indemnified  Party (including  reasonable
fees and expenses to the extent  incurred in connection  with  investigating  or
preparing  to defend  such  Proceeding  in a manner not  inconsistent  with this
Section) shall be paid to the Indemnified Party,

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<PAGE>


as  incurred,  within  ten  Trading  Days  of  written  notice  thereof  to  the
Indemnifying  Party  (regardless of whether it is ultimately  determined that an
Indemnified Party is not entitled to indemnification  hereunder;  provided, that
the  Indemnifying  Party may require  such  Indemnified  Party to  undertake  to
reimburse  all such fees and  expenses  to the extent it is  finally  judicially
determined  that  such  Indemnified  Party is not  entitled  to  indemnification
hereunder).

          (d) Contribution. If a claim for indemnification under Section 5(a) or
5(b) is  unavailable  to an  Indemnified  Party (by  reason of public  policy or
otherwise),   then  each  Indemnifying  Party,  in  lieu  of  indemnifying  such
Indemnified  Party,  shall  contribute  to the  amount  paid or  payable by such
Indemnified  Party  as a  result  of  such  Losses,  in  such  proportion  as is
appropriate  to  reflect  the  relative  fault  of the  Indemnifying  Party  and
Indemnified  Party in connection with the actions,  statements or omissions that
resulted in such Losses as well as any other relevant equitable  considerations.
The relative fault of such  Indemnifying  Party and  Indemnified  Party shall be
determined by reference to, among other things,  whether any action in question,
including any untrue or alleged untrue  statement of a material fact or omission
or alleged omission of a material fact, has been taken or made by, or relates to
information  supplied by, such Indemnifying  Party or Indemnified Party, and the
parties'  relative intent,  knowledge,  access to information and opportunity to
correct or prevent  such  action,  statement  or  omission.  The amount  paid or
payable by a party as a result of any Losses shall be deemed to include, subject
to the limitations set forth in Section 5(c), any reasonable attorneys' or other
reasonable  fees or  expenses  incurred  by such  party in  connection  with any
Proceeding to the extent such party would have been indemnified for such fees or
expenses if the  indemnification  provided for in this Section was  available to
such party in accordance with its terms.

          The parties  hereto  agree that it would not be just and  equitable if
contribution  pursuant  to  this  Section  5(d)  were  determined  by  pro  rata
allocation or by any other method of allocation  that does not take into account
the equitable considerations referred to in the immediately preceding paragraph.
Notwithstanding the provisions of this Section 5(d), no Holder shall be required
to contribute, in the aggregate, any amount in excess of the amount by which the
proceeds  actually  received  by such  Holder  from the sale of the  Registrable
Securities subject to the Proceeding exceeds the amount of any damages that such
Holder has  otherwise  been  required to pay by reason of such untrue or alleged
untrue statement or omission or alleged omission.

          The indemnity and  contribution  agreements  contained in this Section
are in addition to any liability that the  Indemnifying  Parties may have to the
Indemnified Parties.

     6. Miscellaneous

          (a) Remedies.  In the event of a breach by the Company or by a Holder,
of any of their obligations under this Agreement, each Holder or the Company, as
the case may be, in addition to being entitled to exercise all rights granted by
law and under this Agreement, including recovery of damages, will be entitled to
specific  performance of its rights under this  Agreement.  The Company and each
Holder agree that monetary damages would not provide  adequate  compensation for
any losses incurred by reason of a breach by it of any of the provisions of this
Agreement  and  hereby  further  agrees  that,  in the event of any  action  for

                                      -9-
<PAGE>


specific  performance in respect of such breach, it shall waive the defense that
a remedy at law would be adequate.

          (b) No  Piggyback  on  Registrations.  Except  as  and  to the  extent
specified in Schedule  6(b) hereto,  neither the Company nor any of its security
holders  (other than the Holders in such capacity  pursuant  hereto) may include
securities  of  the  Company  in  the  Registration  Statement  other  than  the
Registrable  Securities,  and the Company  shall not after the date hereof enter
into any  agreement  providing  any such right to any of its  security  holders.
Except as and to the extent  specified in Schedule 6(b) hereto,  the Company has
not previously entered into any agreement granting any registration  rights with
respect  to any of its  securities  to any  Person  which  have not  been  fully
satisfied.

          (c) Compliance.  Each Holder  covenants and agrees that it will comply
with the prospectus delivery requirements of the Securities Act as applicable to
it  in  connection  with  sales  of  Registrable   Securities  pursuant  to  the
Registration Statement.

          (d) Discontinued Disposition. Each Holder agrees by its acquisition of
such  Registrable  Securities that, upon receipt of a notice from the Company of
the occurrence of any event of the kind  described in Section 3(c),  such Holder
will forthwith discontinue  disposition of such Registrable Securities under the
Registration  Statement  until  such  Holder's  receipt  of  the  copies  of the
supplemented  Prospectus  and/or amended  Registration  Statement or until it is
advised in writing (the  "Advice") by the Company that the use of the applicable
Prospectus  may be resumed,  and, in either  case,  has  received  copies of any
additional  or  supplemental  filings  that are  incorporated  or  deemed  to be
incorporated  by reference in such  Prospectus or  Registration  Statement.  The
Company may provide  appropriate  stop orders to enforce the  provisions of this
paragraph.

          (e) Piggy-Back Registrations.  If at any time during the Effectiveness
Period  there is not an  effective  Registration  Statement  covering all of the
Registrable  Securities and the Company shall determine to prepare and file with
the  Commission  a  registration  statement  relating to an offering for its own
account or the account of others under the  Securities  Act of any of its equity
securities,  other than on Form S-4 or Form S-8 (each as  promulgated  under the
Securities Act) or their then  equivalents  relating to equity  securities to be
issued solely in connection  with any  acquisition  of any entity or business or
equity  securities  issuable in connection  with stock option or other  employee
benefit plans, then the Company shall send to each Holder written notice of such
determination and, if within fifteen days after receipt of such notice, any such
Holder  shall  so  request  in  writing,  the  Company  shall  include  in  such
registration  statement  all or any  part of such  Registrable  Securities  such
holder  requests to be  registered,  subject to customary  underwriter  cutbacks
applicable to all holders of registration rights.

          (f)  Amendments  and  Waivers.   The  provisions  of  this  Agreement,
including  the  provisions  of this  sentence,  may not be amended,  modified or
supplemented,  and waivers or consents to departures from the provisions  hereof
may not be given,  unless the same shall be in writing and signed by the Company
and  the  Holders  of  all  of  the  then  outstanding  Registrable  Securities.
Notwithstanding the foregoing, a waiver or consent to depart from the provisions
hereof  with  respect  to a matter  that  relates  exclusively  to the rights of
certain  Holders and that

                                      -10-
<PAGE>



does not directly or indirectly  affect the rights of other Holders may be given
by Holders of at least a majority of the  Registrable  Securities  to which such
waiver or consent  relates,  provided,  that the provisions of this sentence may
not be  amended,  modified,  or  supplemented  except  in  accordance  with  the
provisions of the immediately preceding sentence.

          (g) Notices. Any and all notices or other communications or deliveries
required or permitted to be provided  hereunder shall be in writing and shall be
deemed given and effective on the earliest of (i) the date of  transmission,  if
such  notice or  communication  is  delivered  via  facsimile  at the  facsimile
telephone  number  specified in this Section  prior to 6:30 p.m.  (New York City
time) on a Trading Day, (ii) the Trading Day after the date of transmission,  if
such  notice or  communication  is  delivered  via  facsimile  at the  facsimile
telephone number specified in this Agreement later than 6:30 p.m. (New York City
time) on any date and earlier than 11:59 p.m. (New York City time) on such date,
(iii) the  Trading Day  following  the date of  mailing,  if sent by  nationally
recognized  overnight courier service,  or (iv) upon actual receipt by the party
to whom such notice is required  to be given.  The address for such  notices and
communications shall be as follows:

         If to the Company:         Aphton Corporation
                                    [ ]
                                    Attn: [ ]
                                    Fax No.: [ ]

         With a copy to:            [ ]
                                    Attn:  [ ]
                                    Facsimile No.: [ ]

         If to a Purchaser:         To the address set forth
                                    under the Purchaser's name on the signature
                                    page hereto.

         With a copy to:            Bryan Cave LLP
                                    1290 Avenue of the Americas
                                    New York, NY 10101
                                    Attn.: Eric L. Cohen, Esq.
                                    Fax No.: (212) 541-4630 and (212) 541-1432

         If to any other Person who is then the registered Holder:

                                    To the address of such Holder as it appears
                                    in the stock transfer books of the Company

or such other  address as may be designated  in writing  hereafter,  in the same
manner, by such Person.

          (h) Successors and Assigns.  This Agreement shall inure to the benefit
of and be  binding  upon the  successors  and  permitted  assigns of each of the
parties  and shall  inure to the  benefit of each  Holder.  The  Company may not
assign its rights or obligations  hereunder without the prior written consent of
each Holder.  Each Holder may assign their  respective  rights  hereunder in the
manner and to the Persons as permitted under the Purchase Agreement.

                                      -11-
<PAGE>


          (i) Execution and Counterparts.  This Agreement may be executed in any
number of counterparts,  each of which when so executed shall be deemed to be an
original  and, all of which taken  together  shall  constitute  one and the same
Agreement.   In  the  event  that  any   signature  is  delivered  by  facsimile
transmission,  such  signature  shall create a valid  binding  obligation of the
party  executing  (or on whose behalf such  signature is executed) the same with
the same  force and  effect as if such  facsimile  signature  were the  original
thereof.

          (j)  Governing  Law.  All  questions   concerning  the   construction,
validity,  enforcement and interpretation of this Agreement shall be governed by
and construed and enforced in accordance  with the internal laws of the State of
New York,  without  regard to the  principles of conflicts of law thereof.  Each
party  agrees  that  all  legal  Proceedings   concerning  the  interpretations,
enforcement  and  defense of the  transactions  contemplated  by this  Agreement
(whether brought against a party hereto or its respective affiliates, directors,
officers, shareholders, employees or agents) shall be commenced in the state and
federal courts sitting in the City of New York, Borough of Manhattan. Each party
hereto hereby irrevocably submits to the exclusive jurisdiction of the state and
federal  courts  sitting in the City of New York,  Borough of Manhattan  for the
adjudication  of any dispute  hereunder  or in  connection  herewith or with any
transaction  contemplated  hereby or discussed herein (including with respect to
the enforcement of this Agreement),  and hereby  irrevocably  waives, and agrees
not to assert  in any  suit,  action  or  Proceeding,  any claim  that it is not
personally subject to the jurisdiction of any such court, that such suit, action
or Proceeding is improper. Each party hereto (including its affiliates,  agents,
officers,  directors and employees)  hereby  irrevocably  waives, to the fullest
extent  permitted by  applicable  law, any and all right to trial by jury in any
legal  Proceeding   arising  out  of  or  relating  to  this  Agreement  or  the
transactions  contemplated  hereby.  If either party shall commence an action or
Proceeding to enforce any  provisions  of this  Agreement,  then the  prevailing
party in such action or  Proceeding  shall be  reimbursed by the other party for
its attorneys fees and other costs and expenses incurred with the investigation,
preparation and prosecution of such action or Proceeding.

          (k) Cumulative  Remedies.  The remedies provided herein are cumulative
and not exclusive of any remedies provided by law.

          (l) Severability.  If any term, provision,  covenant or restriction of
this  Agreement  is held by a court of  competent  jurisdiction  to be  invalid,
illegal,  void  or  unenforceable,  the  remainder  of  the  terms,  provisions,
covenants  and  restrictions  set forth  herein  shall  remain in full force and
effect and shall in no way be affected, impaired or invalidated, and the parties
hereto  shall use their  reasonable  efforts to find and  employ an  alternative
means to achieve the same or substantially  the same result as that contemplated
by such term,  provision,  covenant or restriction.  It is hereby stipulated and
declared to be the  intention of the parties  that they would have  executed the
remaining terms, provisions, covenants and restrictions without including any of
such that may be hereafter declared invalid, illegal, void or unenforceable.

          (m) Headings.  The headings in this  Agreement are for  convenience of
reference only and shall not limit or otherwise affect the meaning hereof.

                   [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK
                           SIGNATURE PAGES TO FOLLOW]
<PAGE>




          IN WITNESS WHEREOF, the parties have executed this Registration Rights
Agreement as of the date first written above.

                                            APHTON CORPORATION

                                            By:  /s/ Philip C. Gevas
                                               ---------------------------------
                                            Name:  Philip C. Gevas
                                            Title:  Chief Executive Officer



                   [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK
                     SIGNATURE PAGE OF PURCHASER TO FOLLOW]
<PAGE>


          IN WITNESS WHEREOF, the parties have executed this Registration Rights
Agreement as of the date first written above.


                               MAINFIELD ENTERPRISES INC.


                               By:  /s/ Kenneth L. Henderson
                                  -------------------------------------
                               Name:  Kenneth L. Henderson
                               Title:  Attorney-in-fact

                               Address for Notice:

                               [ ]
                               Facsimile No.: [ ]
                               Attn: [ ]

                               With a copy to:

                               Bryan Cave LLP
                               1290 Avenue of the Americas
                               New York, NY  10104
                               Facsimile No.:  (212) 541-4630 and (212) 541-1432
                               Attn:  Eric L. Cohen, Esq.
<PAGE>




                                                                         Annex A

                              Plan of Distribution

     The  Selling  Stockholders  and  any  of  their  pledgees,   assignees  and
successors-in-interest  may, from time to time,  sell any or all of their shares
of Common Stock on any stock exchange,  market or trading  facility on which the
shares  are traded or in private  transactions.  These  sales may be at fixed or
negotiated  prices.  The  Selling  Stockholders  may  use any one or more of the
following methods when selling shares:

     $    ordinary   brokerage   transactions  and  transactions  in  which  the
          broker-dealer solicits purchasers;

     $    block  trades  in which the  broker-dealer  will  attempt  to sell the
          shares as agent but may  position and resell a portion of the block as
          principal to facilitate the transaction;

     $    purchases  by  a   broker-dealer   as  principal  and  resale  by  the
          broker-dealer for its account;

     $    an  exchange   distribution  in  accordance  with  the  rules  of  the
          applicable exchange;

     $    privately negotiated transactions;

     $    short sales

     $    broker-dealers  may  agree  with the  Selling  Stockholders  to sell a
          specified number of such shares at a stipulated price per share;

     $    a combination of any such methods of sale; and

     $    any other method permitted pursuant to applicable law.

     The  Selling  Stockholders  may also sell  shares  under Rule 144 under the
Securities Act, if available, rather than under this prospectus.

     Broker-dealers  engaged by the Selling  Stockholders  may arrange for other
brokers-dealers to participate in sales.  Broker-dealers may receive commissions
or discounts from the Selling  Stockholders  (or, if any  broker-dealer  acts as
agent  for the  purchaser  of  shares,  from the  purchaser)  in  amounts  to be
negotiated.  The  Selling  Stockholders  do not  expect  these  commissions  and
discounts to exceed what is customary in the types of transactions involved.

     The  Selling  Stockholder  may from time to time pledge or grant a security
interest  in some or all of the common  stock or Warrant  owned by them and,  if
they default in the  performance of their secured  obligations,  the pledgees or
secured  parties may offer and sell the shares of common stock from time to time
under this  prospectus,  or under an  amendment  to this  prospectus  under Rule
424(b)(3) or other  applicable  provision of the Securities Act of 1933
<PAGE>



amending the list of selling stockholders to include the pledgee,  transferee or
other successors in interest as selling stockholders under this prospectus.

     The Selling  Stockholders  also may  transfer the shares of common stock in
other circumstances, in which case the transferees, pledgees or other successors
in  interest  will  be the  selling  beneficial  owners  for  purposes  of  this
prospectus.

     The Selling Stockholders and any broker-dealers or agents that are involved
in selling the shares may be deemed to be  "underwriters"  within the meaning of
the Securities Act in connection with such sales. In such event, any commissions
received  by such  broker-dealers  or agents and any profit on the resale of the
shares  purchased  by them  may be  deemed  to be  underwriting  commissions  or
discounts under the Securities Act. The Selling  Stockholders  have informed the
Company  that it does not have  any  agreement  or  understanding,  directly  or
indirectly, with any person to distribute the Common Stock.

     The  Company  is  required  to pay all fees and  expenses  incident  to the
registration  of the shares.  The Company  has agreed to  indemnify  the Selling
Stockholders against certain losses, claims, damages and liabilities,  including
liabilities under the Securities Act.